tengo un error alguien me pudiera ayudar de faor

Fatal error: Uncaught Error: Call to undefined function mysql_query() in C:\xampp\htdocs\prueba\registro.php:16 Stack trace: #0 {main} thrown in C:\xampp\htdocs\prueba\registro.php on line 16

este es el codigo

<?php
$db_host="localhost";
$db_user="root";
$db_password="";
$db_name="prueba";
$db_table_name="datos";
$db_connection = mysqli_connect($db_host, $db_user, $db_password);
 
if (!$db_connection) {
	die('No se ha podido conectar a la base de datos'.mysqli_error());
}
$subs_name = utf8_decode($_POST['nombre']);
$subs_last = utf8_decode($_POST['apellido']);
$subs_email = utf8_decode($_POST['email']);
 
$resultado = mysql_query("SELECT * FROM ".$db_table_name." WHERE Email = '".$subs_email."'", $db_connection);
 
 
if (mysql_num_rows($resultado)>0)
{
 
header('Location: Fail.html');
 
} else {
 
	$insert_value = 'INSERT INTO `' . $db_name . '`.`'.$db_table_name.'` (`Nombre` , `Apellido` , `Email`) VALUES ("' . $subs_name . '", "' . $subs_last . '", "' . $subs_email . '")';
 
mysql_select_db($db_name, $db_connection);
$retry_value = mysql_query($insert_value, $db_connection);
 
if (!$retry_value) {
   die('Error: ' . mysql_error());
}
 
header('Location: Success.html');
 
}
 
mysql_close($db_connection);
 
 
?>

MOD: Etiqueta GeSHi.